Suspended boom sprayers are commonly used in agriculture in order to apply materials such as pesticides, herbicides and fertilizers over a wide swathe of land. By way of an example, FIG. 1 shows a schematic representation of the components commonly found in a conventional sprayer boom 10, including the two boom wings 11, 12, a centre frame 13 to which the boom wings 11, 12 are connected and a vehicle frame 14 to which the centre frame 13 is attached. Common sprayer boom designs use a boom centre frame 13 with wings 11, 12 suspended from each side. The wings of variable geometry booms can be adjusted upward or downward using one or more hydraulic actuators 15, each wing 11, 12 pivoting from a hinge point 16 connection with the centre frame 13. These booms may also be folded inwardly for transport with hydraulic actuators, pivoting each wing 11, 12 about a vertical axis.
In order to efficiently cover a large swathe of ground, it is advantageous to use boom sprayers with long wings. The boom wings from which the materials are sprayed can often extend to more than 22 meters from the chassis of a vehicle upon which the boom is mounted. As a result of this large distance, a small rotation of the chassis can cause a significant movement of the ends of the boom wings. Consequently the spray tips deviate from their intended location, which can cause the spray to be applied unevenly across the field. There is also a possibility of the boom wings crashing into the ground or another obstacle. Furthermore, the acceleration required to move the ends of the boom wings produces large stresses along the length of the wings, which can damage or break them.
It is therefore necessary for vehicles with long sprayer booms to include a means of mitigating the effects of chassis rotation on the position of the boom wings. This is typically achieved using a suspension system, a positioning system, or a combination of the two.
Rotational movement of the boom is usually described in terms of roll, yaw and pitch, which correspond to rotation around three different axes. For the purposes of the present invention, pitch motion is less important than roll and yaw so this will not be a point of emphasis herein.
Rolling movement corresponds to a rotation around an axis parallel to the direction that the vehicle travels in. In some designs this can occur when the wheels on only one side of the vehicle travel over a bump in the ground or when spraying a sloped field. The extent to which this effect is seen will be dependent upon many factors such as the relative positions of the centre of gravity and the boom pivot point, as well as the sprayer chassis suspension, tyres, chassis flex, and so on. Rolling rotation causes up-down movement of the boom wings and therefore creates a significant risk of damage to the boom from striking the ground as well as resulting in uneven spray application.
Yawing movement corresponds to rotation around a vertical axis and occurs when the vehicle turns. This causes a forward-back motion of the boom wings, which is unlikely to result in the wings striking the ground but still produces large stresses and causes uneven chemical application.
The effect of boom yaw may also transfer into a boom roll component, and the effect of boom roll may also transfer into a boom yaw component.
From now onwards rolling and yawing motion will be treated herein as interchangeable, unless they are specifically described as otherwise, with the described displacements being applicable to rotations around either of the relevant axes.
An important aspect of sprayer design is a suspension that uncouples the spray boom from the chassis to provide a uniform spray distribution. These systems reduce the effect of input disturbances on the boom such as field bumps causing chassis roll, or steering/tracking input producing chassis yaw, which affect boom stability and consequently application performance.
When a spray vehicle is driving over uneven terrain, any disturbances will usually be imparted to the vehicle through its wheels, and must then be transmitted through a number of elements before reaching the boom wings. There are, therefore, a number of different locations at which the individual boom wings may be uncoupled from an outside disturbance input.
FIG. 2 shows a schematic diagram of a general suspension system in which each of the main elements of a boom sprayer vehicle is uncoupled to some extent from its surrounding components by use of parallel spring and damping components. In FIG. 2:                Level inclination/reference distance 20 is the distance from a specified reference point, which could, for certain systems, be the horizontal plane normal to gravity;        The disturbance input 21 is the effect between the ground and vehicle tyres, such as a bump beneath a wheel;        22 and 23 are the spring and damping system component of the tyres and axle suspension;        24 is the rotational inertia of the sprayer chassis;        25 is a support frame of the boom suspension;        26 is a hydraulic actuator between the support frame 25 and an intermediate frame 27 which will exist in some common designs;        28 and 29 are the spring and damper components of a pivoting boom suspension system. The spring and damper locations can be reversed without affecting the operation of the system, and the suspension (spring damper) and roll actuator can be reversed as well;        30 is the boom frame;        31 is the pendulum centring force applied to the boom frame. This is likely not applicable for yaw suspension systems because boom yaw is not typically affected by gravity;        32 is the rotational inertia of the boom frame, which is typically small in comparison to the left and right wings;        33 and 34 are the spring and damper for a linked boom roll suspension (if applicable);        40 and 50 are the hydraulic actuators that connect between the boom center frame 30 and the left and right wing boom section. These are normally used to adjust the position of the wings with respect to the centre frame to follow changing terrain;        41 and 51 represent the rotational inertia of the left and right boom wings respectively;        42 and 52 are the spring components of the left and right wing independent suspension; and        43 and 53 are the damping components of the left and right wing independent suspension.        
Suspension elements 28, 29 and 33, 34 are not likely to occur in the same system because they are basically redundant. 28/29 is found in a pivoting suspension, whereas 33/34 is found in a linked suspension.
Most common suspension designs usually include some but not all of these components. The pendulum-centring force 31, for instance, is due to gravity and may be found in roll suspension systems but is unlikely to be applicable to a yaw suspension system. In a roll suspension system, such a term would depend on the centre of gravity of the boom wings and the location of their pivot points. Not all roll suspension systems will have this term.
Some conventional boom height control systems use boom suspension position as an input for a controller. The method of measurement may be called the “displacement method” wherein suspension displacement is measured using a relative position (or angular) measurement between two points of the suspension. The suspension displacement may be measured and used to optimize the automated corrections made to wing position. In some instances suspension deflection is measured at multiple points because more than one source of suspension may exist. This measurement may also require compensation to correct for static offsets in suspension position due to gravitational effects on sidehills or radial acceleration from driving around corners.
Boom suspension displacement measurements may be used to determine the torque existing between the sprayer chassis and boom in order to minimize the torque using a torque biasing element, such as a mechanical system in series with the boom suspension. Force is measured as a function of suspension or spring displacement.
These conventional methods may face the following challenges:                (1) Displacement is measured at the primary point of suspension deflection; however some amount of suspension deflection will almost always occur in other points of the sprayer. This may include the sprayer tyres, axle suspension, frame flex, and lift arm flex, for example. The extent of this problem will vary from one machine design to the next. Some designs may also feature multiple points of designed boom rotation requiring multiple measurements;        (2) Some sprayer designs do not feature boom roll suspension and consequently suspension is occurring in other places, such as twist on the chassis. These sprayer designs may be referred to as rigid booms. In this situation it is difficult to measure suspension activity as a function of deflection using conventional methods;        (3) Different sprayer designs require a variety of sensor and mounting options in order to measure suspension accurately increasing system complexity; and        (4) The boom suspension typically includes a damping element which will dampen suspension deflection and therefore attenuate force measurements, reducing measurement sensitivity.        
Therefore, a need exists for an improved technique for a control system for a suspended boom sprayer of a vehicle.